This invention relates to coatings for forming resistive or conductive thick films such as for resistance heating and electrode applications.
The expressions xe2x80x9cresistivexe2x80x9d and xe2x80x9cconductivexe2x80x9d in association coatings are herein used to refer to coatings which will pass electrical currents as opposed to those which are insulative. Whether a coating is deemed resistive or conductive depends on how freely it passes an electrical current. The expression xe2x80x9celectrically conductive coatingxe2x80x9d is intended to include both resistive and conductive coatings.
Previous thick film conductive and resistive coatings have been either epoxy-based or glass-based. The epoxy-based coatings generally included silver, nickel or carbon as a conductive material and in some instances chromium. A limitation to epoxy based coatings is that the epoxy (or other polymeric binder) is limited in temperature capability and therefore not suitable for many resistive element applications such as kettles or stove top elements. Typically the epoxy will degrade at a temperature of 200xc2x0 C. or less.
Glass based conductive or resistive coatings employ an organic vehicle, a glass binder and a functional phase. The functional phase consists of metal particles such as silver, silver-palladium, copper or nickel, or semiconducting oxide particles such as ruthenium dioxide, bismuth ruthenate, lead ruthenate or bismuth iridate. Two significant technical limitations are encountered in using glass-based thick film resistor and conductor materials. Firstly, the films are typically deposited at a processing temperature in excess of 800xc2x0 C. in order to process the film, precluding the use on substrate materials requiring lower temperatures. Glass frits with lower firing temperature are available, but typically contain a significant amount of lead which is undesirable in many applications due to its toxic properties. Secondly, the thermal expansion coefficient of the glass matrix must be similar to that of the substrate material in order to obtain sufficient film adhesion. Mismatched thermal expansion coefficients will result in spalling of the film upon initial cooling or in subsequent use.
In view of the limitations of processing temperature and matching of thermal expansion coefficients, some substrate materials such as aluminum are not feasible due to temperature limitations or lack of a compatible glass matrix. Additionally, this technology requires expensive semimetallic and metallic particle phase materials.
Another method of applying conductive and resistive coatings is through the chemical vapour deposition (xe2x80x9cCVDxe2x80x9d) of thin film tin oxide-based resistive heating elements. This technology involves spraying a mist of stannic chloride onto the substrate when the substrate temperature is at 550xc2x0 C. to produce thin layers of less than 1 micron in thickness. Resistive films produced by this technology are limited in the temperature of operation (at 350xc2x0 C. the resistance increases), and in the substrates on which they can be deposited. The deposition temperature and the low thermal expansion coefficient (xe2x80x9cCTExe2x80x9d) of the resulting film limit the use of this technology to substrate materials with low CTE. It is not possible to deposit this layer on aluminum based substrates as thermal expansion differences eventually produce microcracks in the thin film. Another limitation of this technology is that it can only be deposited on materials with smooth surface morphologies.
One particular application has brought to light the shortcomings of state of the art resistive coatings for use as heating elements. The particular application relates to integrated heating elements for what are referred to as xe2x80x9chot topxe2x80x9d glass based stoves. Current technology uses a resistive coil or heat lamp that is placed below the glass to provide the heat. Efforts to replace this design with an integrated heating design have proved unsuccessful. The glass is of a special composition having virtually zero thermal expansion and is not readily coated using glass based coatings because of processing temperatures and adhesion problems. Epoxy-based coatings are not a suitable alternative as they will not withstand the service requirements requiring temperatures of around 400xc2x0 C.
Attempts have been made to use a CVD process to deposit a resistive element on the glass-ceramic. While the CVD resistive element has been deposited successfully on this material, the glass-ceramic becomes conductive at 400xc2x0 C. and therefore, as such cannot meet the requirements of the European electrical safety standards for appliances (less than 100 mA at 3,750 V AC at operating temperature for 60 seconds). Accordingly a sol-gel composite insulating layer based on aluminum oxide or aluminum nitride has been provided between the glass and the resistive coating. However, the deposited layer typically has a surface roughness greater than the thickness of the CVD deposited coating which prevents the formation of a suitable resistive element from the CVD process.
It is an object of the present invention to provide a conductive or resistive coating which may be easily applied such as by being spray, dip, spin, brush or screen-print deposited without requiring vapour deposition techniques, which doesn""t require high forming temperatures, and which can be produced to have desired thermal expansion properties.
It is a further object of the present invention to provide such a coating which may be effectively applied to a porous substrate and which is relatively insensitive to the shape of the substrate.
A composition is provided for application to a substrate to form an electrically conductive coating thereon. The composition includes a sol-gel solution in which up to about 90% of the solution is a mixture of conductive and insulative powders in a uniform stable dispersion. The conductive particles may be metallic, ceramic, inter-ceramic or semi-conductors. The insulative particles may be metal oxide or ceramic.
The conductive particles may be at least one of a carbide, nitride, boride, silicide, oxide, graphite, molybdenum, tungsten, tantalum, nickel, chromium, silver, silver-palladium alloy, iron-nickel-chromium alloy, nickel chromium alloy, or iron-chromium-aluminum alloy. Preferably the conductive particles will have a particle size in the range of 1 to 100 microns but more preferably 2 to 20 microns.
A process is provided for producing a resistive or conductive coating on a substrate which comprises the steps of:
a. mixing a sol-gel solution with a conductive powder selected from the group consisting of ceramics, inter-ceramics, semi-conductors and metals so as to produce a uniform stable dispersion;
b. applying said stable dispersion to a substrate, so as to provide a coating thereon; and
c. firing said coated substrate at a temperature sufficient to remove organic constituents and produce an at least partially conductive film on said substrate.
Steps b and c may be repeated as necessary to produce a stable coating of a desired thickness.
An insulative ceramic powder may be incorporated into the system to alter the resistance of the deposited layer. Possible candidates include but are not limited to aluminum oxide, silicon oxide, barium titanate, silicon carbide and iron oxide.
The sol-gel solution may be selected from the group including aluminates, silicates, titanates, zirconates or combinations thereof.
A heating device is provided which has a substrate of glass, metal or ceramic and a sol-gel derived resistive heating layer coated on a heated face of the substrate.
A heating device is further provided which has a contact member with a heating face opposite a heated face. An electrically insulative layer is bonded to the heating face and has an outer face distal the heated face. A sol-gel derived resistive heating layer is coated on the outer face of the electrically insulative layer. The sol-gel derived resistive heating layer may be in accordance with the compositions set out above and applied to the insulative layer according to the process also described above.